In a typical case, the borehole for an oil well prior to completion of the well has casing set therein and extending through the depth zone in which the oil bearing formation lies, with cement in place between the casing and the formation. To complete the well, some means is provided to perforate the casing and cement and, to some extent, the formation, so as to permit flow of oil from the formation into the casing (or tubing) in which it is carried to the surface.
The perforating is accomplished by apparatus including one or more perforating guns which are suspended in the casing (or tubing) in the zone to be perforated. Each gun carries a plurality of perforator units, which conventionally are either bullets or shaped charges.
It is desirable that the perforator unit should penetrate as far as possible into the formation and create the best possible environment for the flow of oil from the formation into the casing (or tubing).
In order to enhance the effectiveness of perforations, it has been proposed in the prior art to utilize in each perforator unit the combination of a shaped charge and a bullet. However, prior art attempts to effectively utilize such combination have not proved successful. In U.S. Pat. No. 2,946,283, which exemplifies such prior art attempts, the bullet charge is first ignited, so that the bullet moves forward and strikes a detonator cap which then ignites the shaped charge. This arrangement does not function satisfactorily since the bullet has a strong tendency to interfere with the action of the shaped charge and vice versa. The result is a negative effect instead an enhancement.
The object of this invention is to provide a method and apparatus wherein each perforator unit of a perforator gun utilizes the combination of a shaped charge and a bullet in such manner as to significantly enhance the effectiveness of resulting perforations.
For a further understanding of the invention and further objects, features, and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawing.